This grant proposes to study the relationships between DNA synthesis in bone cells, alterations in the morphology of osteoclasts and bone resorption. The osteoclast is the principal cell which mediates bone resorption. Osteoclasts increase their size and number in bone by incorporating a terminally differentiated cell, the osteoclast precursor. Production of osteoclast precursor cells is believed to be dependent on the replication and differentiation of a stem cell. The projects in this grant will attempt to answer the questions: 1) Do factors which alter bone resorption affect the replication of the osteoclast precursor stem cells? 2) Do factors which regulate the replication of osteoclast precursor stem cells alter bone resorption? and 3): What is the relationship between inhibiting cell replication in bone cells and the production of prostaglandins by these cells. I will study fetal rat long bone cultures for the effects that known regulators of bone resorption have on DNA synthesis, osteoclast morphology and the incorporation of recently replicated nuclei into osteoclasts. Bones will be cultured with inhibitors of DNA synthesis to determine if alterations of cell replication influence the resorptive response. I will also determine the effects that inhibitors of DNA synthesis have on the differentiation of cells in organ and cell cultures and on the concentration of prostaglandins in the medium since I have preliminary evidence linking increases in medium prostaglandin concentrations with the inhibition of cell replication. I will examine conditioned media which contain granulocyte-monocyte colony stimulating factors to determine if they alter bone resorption rates or osteoclast morphology. I will determine the similarities between the responses of fetal rat long bone cultures to known stimulators of resorption and the responses to conditioned media from tumor cell cultures which, in vivo, produce humoral hypercalcemia. Finally, I will attempt to develop a cell culture system which allows rat bone marrow cells to replicate and differentiate into osteoclast-like cells.